This invention relates to a vacuum pump which has a device for measuring the rpm of at least one pump piston or pump rotor. The shaft of the piston or rotor is supported in a bearing member (bearing plate or bearing block) and the terminus of the shaft projects into a side chamber of the pump.
Lobed rotary piston pumps (Roots pumps) constructed according to current technology have a split tube motor drive and are completely encapsulated so that all rotary components are accommodated within the pump housing. Further, the side chambers of the Roots pumps are, as a rule, under vacuum as the pump operates. Roots pumps are often used to drive corrosive fluids whose penetration into the side chambers cannot be prevented. A direct measurement of the rpm by means of a mechanical coupling with the shaft stubs is therefore feasible only with difficulty. Even in case of turbomolecular vacuum pumps, the rotor shaft usually terminates in a chamber, for example, a motor chamber in which usually a pre-vacuum (fore-vacuum) prevails during the operation of the pump.